The invention relates generally to the keys and keypads of operating interfaces of electrical devices. Particularly the invention relates to a thin and simple mechanical key and keypad structure.
A key or a keypad comprises a mechanical structure and an electrical coupling, and this combination transforms the keystrokes made by a user into electrical input signals to the device being used. In many devices the mechanical structure of the keypad currently occupies substantially more space and is much heavier than the electrical coupling with which the keystrokes are detected.
Particularly in the design of many portable terminals the object is to minimise the mass and volume of the device, so that the objects are mechanical structures which are lighter and smaller than previously. However, the design of the structures must observe the fact that particularly the characteristics of the keypad are important regarding the operating convenience, and that for instance too small keys located too close to each other are inconvenient to use. The operating convenience is not directly affected when the keypad structure is made thinner, if this does not essentially impair the tactile feel.
In prior art keypads the electrical coupling which transforms the user""s keystrokes into electrical input signals is generally based on the following idea. Below the keypad having keys in N rows and M columns there are N conductors in the row direction and M conductors in the column direction, so that under each key two conductors cross each other at right angles. When a key is up in the idle position there is no electrical contact between the conductors, but when the user depresses the key an electrical contact is formed between the conductors. The depressed key can be unanimously determined based on the position where said contact is located.
In the electrical couplings of the keypad structures in general use a keystroke should thus cause an electrical contact between adjacent conductors. In prior art mechanical structures this is arranged so that there is an electrically conducting raised dome below the key button. When the key is in the idle state the dome is up, and when the key is depressed it presses the dome down. The dome is pressed against the crossing conductors below it, and an electrical contact is formed between them. The dome acts also as a spring which returns the key into the initial position when the force acting from above on the key is removed. Thus a separate spring below the key button is not required. For the sake of clarity is should be mentioned that below the term xe2x80x9ckeyxe2x80x9d refers to that key button which the user is striking. The terms mechanical or electrical structure of a key or keypad refer also to other parts of the key than to only to the key button.
The mechanical structure of a prior art key is shown in FIG. 1. On the printed circuit board 101, which comprises a circuit arrangement for reading the keystrokes, there is an electrically conducting dome 102 in the intersection of the conductors. If the whole dome is made of an electrically conducting material, typically of metal, then the edges of the dome can be isolated from the conductors, for instance by a separate isolating layer 103 having at the conductor intersections holes with a diameter smaller than that of the domes, or by using a multi-layer printed circuit board whereby the conductors are on the surface of the board only at their intersections. If only that part of the dome 102 which in a keystroke is pressed against the printed circuit board is of an electrically conducting material, then a separate isolating layer 103 is not required, as the dome can rest against the printed circuit board supported directly by its electrically isolating periphery. Such a dome with a lower surface conducting in only some places can be made for instance by metalling a suitable part of the concave lower surface of a dome made of plastics. Over each dome there is a key 104 having a cylindrical upper part. The lower part of the key expands into a cylindrical base containing a hollow which enables the normal upper position of the dome when the key is not depressed. Thus the key rests against the printed circuit board 101 on a peripheral support surface.
In the bottom of the key""s upper part, against the dome, there is a projection 105 having a smaller area than the upper part and actually depressing the electrically conducting centre of the dome against the printed circuit board when the key is depressed. The structure formed by the cylindrical upper part of the key and the projection at the bottom of the key is substantially rigid, and when the key is depressed the elasticity enabling the vertical movement of the key occurs at the thin joint between the upper part and the base. The material of the key must be sufficiently flexible so that the joint between the base and the upper part will yield in a desired manner. The keys can be made of rubber, for instance.
The projection in the bottom of the cylindrical upper part of the key is necessary, because otherwise the whole bottom surface of the key""s upper part would be pressed against the electrically conducting dome. Then the spring-like action of the dome when it is depressed by the narrower point could be lost, and the force required to press the button so that the whole surface of the dome could be pressed against the conductors on the surface of the printed circuit board would be greater than in the structure shown in FIG. 1.
A prior art way to fasten a key or a keypad to the electrical device in question is shown in FIG. 2. The keys, which are either separate or form a continuous keymat by being fastened to each other at their bases, move vertically in the guiding gaps of the cover 201. The cover is fastened at its edges to the frame of the device, whereby it presses the keys/keypad against the electrically connecting domes and the printed circuit board. A generally used cover profile is seen in FIG. 2: in order to minimise the material used for the cover and the mass of the cover it is not a continuous perforated plate with an even thickness.
A prior art keypad is assembled of many separate components, so the assembly requires several work steps. The thickness of the keypad is several millimetres, typically 5 to 8 mm. Particularly in small portable terminals this thickness is considerable compared to the total thickness of the device. If the keypad is designed to have a convex form, for instance, then the cylindrical parts of the keys and the cover at the centre of the keypad are higher than at the edges, as the printed circuit board at the bottom of the keypad is generally a planar board.
The operating convenience of the devices increases substantially if they are water-tight, but because in prior art solutions the bottom surface of the keypad cover is uneven, and because the key structure is flexible exactly at that point where the cover rests against the keys/keymat it is difficult to obtain a water-tight structure.
An object of the invention is to present a mechanical key structure which is thin and which enables the construction of thin keypads. Advantageously the new mechanical structure does not require any changes in the electrical couplings of prior art keypads. A further object of the invention is to simplify the structure and production of the keypads.
The object of the invention is attained by coating the electrically conducting domes being an essential part of the keypad structure with a thin dielectric layer.
A key structure according to the invention is a key structure used in the user interface of an electrical device and it comprises
a plate-like member comprising electrical couplings for transforming keystrokes into electrical signals,
an electrically conducting dome-like member on said plate-like member, whereby the dome-like member is arranged to have such suitable elastic characteristics that it is at least partly pressed against the plate-like member when a pressing force substantially perpendicular towards the plate-like member acts on the dome-like member, and to return to the dome-like position when the pressing force is removed, and
a button member made of an electrically isolating material and being located on said dome-like member, wherein said button member is arranged to undergo an elastic deformation due to said pressing force, substantially in the same way as said dome-like member.
The invention relates also to a keypad of the user interface of an electrical device, the keypad comprising
a plate-like member which comprises electrical couplings for transforming keystrokes into electrical signals,
electrically conducting dome-like members on said plate-like member, whereby each of the dome-like members is arranged to have such suitable elastic characteristics that it is at least partly pressed against the plate-like member when a pressing force substantially perpendicular towards the plate-like member acts on the dome-like member, and to return to the dome-like position when the pressing force is removed,
button members made of an electrically isolating material and being located on said dome-like members, and
a cover member covering said plate-like member and the surface of said cover member defines a hole at least at the location of each button member, wherein
said button members are arranged to undergo an elastic deformation due to the effect of said pressing force, substantially in the same way as said dome-like members, and
said cover member is a substantially plate-like body with a uniform thickness, with the exception of the holes.
In the key structure according to the invention there is a key button over the electrically conducting dome on a plate-like member, for example, a printed circuit board, and the elastic deformation of the button against the plate-like member due to the action of a pressing force substantially in the vertical direction and its removal is substantially like that of the electrically conducting dome.
The pressing force presses the electrically conducting dome against conductors, which are close to each other under a certain part, in order to form an electrically conducting contact. This deformation is elastic, and the dome returns into its initial, dome-like position when the force is removed. In its normal position the thin key button according to the invention has substantially the form of the dome, and due to a pressing force it experiences a similar elastic deformation as the dome below it: that part of the key button, which is over that dome part being pressed against the printed circuit board, is pressed downwards so that it imitates the movement of the dome. Both the dome and the key return to the upper position when the pressing force is removed. The characteristics felt by the user""s fingers are good, and compared to the higher key buttons the thin keys save space and manufacturing material.
The use of the thin keys according to the invention makes it possible to use thin, plate-like cover structures for the keypads. The thin keys do not require any guide shafts to prevent lateral swaying, and thus the cover can be as thin as the keys. This saves both space and material. Further the manufacture of a plate-like cover is simpler than the manufacture of a profile structure, for instance. A mechanical keypad structure according to the invention does not require any changes in the electrical couplings and related structures of the keypad.
The keys according to the invention can be fastened at their lower edges directly to the surface of the dome and the plate under the dome, as a thin key imitates the movements of the electrically conducting dome. The cover can also be fastened to the same board, and thus a solid structure is obtained. Another possibility is to fasten the thin, flexible keys according to the invention to a plate-like cover, for instance at the edges of the keys to the edges of the holes in the cover. This provides a continuous cover structure which is water-tight, for instance. Further the manufacture of the device is simple, as no separate keys or keypads must be placed into their places during the assembly phase.